One of the most widely viewed tests we conducted this summer was the one we did with Royal Purple synthetic motor oil in a 290 Chaparral. We bought the Royal Purple off the shelf at Pep Boys and there were no RP personnel on site, nor did they have anything to do with the test other than to pay for it – in advance. We ran the boat with new conventional mineral oil, measured the speed and fuel flow, then replaced it with Royal Purple and measured again. The engine tested also ran 3 dBA quieter. No one was more amazed then the BoatTEST.com staff who are a pretty cynical lot when it comes to manufacturers claims for almost anything.

In a BoatTEST.com test of a Chaparral 290 Signature with twin 5.0L Volvo Penta gas engines comparing conventional oil (“CO”) with synthetic Royal Purple oil (“RP”), the miles per gallon fuel flow was 10.7% greater when Royal Purple oil was used on average across the entire RPM range.

BoatTEST recently compared Royal Purple and a standard motor oil in two 5.0L Volvo Penta gas engines with 115 hours on them.

The engines we tested had 115 hours on them and new, standard oil had been in them for just one engine hour before we tested. 10W30 weight was used in both oils. Reciprocal runs were made in the sound, speed was taken from a gps and fuel flow was measured by BoatTEST equipment which was hooked up to the starboard engine. Sound readings were taken at idle at the dock only of the starboard engine. In the table below conventional oil is noted as “CO” and Royal Purple is “RP.” The results of the test are as follow—

Significant Improvement in Fuel Economy...

As you can see, the Chaparral 290 Signature, which was powered by twin 270-hp engines, was able to turn 100 more rpms at WOT when lubricated with the Royal Purple. And it was only at WOT that the relative speeds of the boat were significantly different, which was due in part to the increased rpms.

The average improvement in miles per gallon when the engines were lubricated with Royal Purple was +10.7%. We believe that is a significant and noteworthy improvement in fuel economy. Drop off the highest and lowest measurements and the average remains the same.

Speed Differences...

Several of our readers wonder why the boat didn’t go the same speed at the same rpm settings. The reason is because we were running the boat on Long Island Sound in choppy conditions that were constantly changing, and not in a laboratory with perfectly still water and no wind. The higher the wind and chop, the harder it is for the engines to push the boat and the more prop slippage there is. Even so the differences in average speeds was only .35 mph or 1.6% of the average speed measured from 2000 to 4500 rpms.

Sound Differences...

We measured sound levels at the dock at idle. We recoded 89 dbA for the starboard engine with conventional oil and 86 db(A) when Royal Purple was in the starboard engine. While this is only 3 db(A) less, in fact the engine was a lot quieter because sound readings are not linier. Db is a logarithmic unit, meaning you cannot add and subtract it. An increase of 3 db is in fact a doubling of the strength of the sound. Again, we believe this is a significant difference and one that is actually measureable with the human ear without a meter.

Oil Pressure and Temperature...

Oil pressure appeared to be about the same for both oils during our test, however oil temperature was different. We measured 160 degrees for the conventional oil after the test run and 155 degrees for Royal Purple after the test.

Reader Questions...

Following are answers to a number of questions and comments from our readers—

Q: Did we fuel up again after the first test run with the standard oil?

A: No. The weight difference was de minimis. We used less than 10 gallons of fuel on the first test, representing about 61 lbs., or less than .007% of the boat’s displacement.

Q: What was the difference in noise levels at full throttle and at best cruise?

A: We did not measure sound underway because wind and wave noise would make the readings meaningless for our test purposes.

Q: Was the test paid for and unbiased?

A: Yes, the test was paid for and it was also unbiased. Tests of this nature usually become the property of the company paying for the test and releasing the findings is typically up to that party. In this case, after the test we informed Royal Purple of the results and told them we would publish them.

Q: Did Royal Purple supply the boat and driver?

A: No. BoatTEST.com arranged for the test boat on its own and supplied the mechanic and driver. In fact, we purchased the Royal Purple oil at Pep Boys from their stock and no Royal Purple personnel were on site during the test nor were they involved with the test in any way.